A popular device for handling large amounts of information in a data processing system is the automated cartridge library. These systems store and manage large numbers of cartridges containing magnetic tape on which data is recorded. Typically, an automated cartridge library is comprised of arrays of uniquely identified cells, each cell being formed to contain a single tape cartridge. The cell arrays each hold a plurality of tape cartridges, and each tape cartridge has some kind of identifying information, such as a bar code. A robotic arm, having an optical system for selecting the correct cartridge, is operable within the automated cartridge library to locate a particular cell, retrieve a tape cartridge therefrom, transport the tape cartridge to a tape drive, and insert the tape cartridge into the tape drive. The tape drive is then operable to read/write data from/to the magnetic tape of the tape cartridge. In this way, a large number of tape cartridges are automatically accessible to a single tape drive mechanism. The operation of the automated cartridge library is typically controlled by a host computer communicating with a library control unit associated with the library.
In some applications, such a large amount of data must be stored that it is desirable to employ more than one automated cartridge library in order to manage the large number of tape cartridges. Rather than repeating an entire automated cartridge library system including all the necessary control units, the host computer, and the tape drive, it is desirable to link together multiple library storage modules. Each library storage module contains cell arrays and a robotic arm as described above, but does not require an additional host computer or tape drive. Multiple library storage modules are arranged adjacent one another and pass-through systems are provided for transferring tape cartridges from one library storage module to an adjacent library storage module. In this arrangement, the host computer can direct a library storage module to pass a tape cartridge to an adjacent library storage module, where it is picked up by the robotic arm of the adjacent library storage module and transported to, and inserted in, the tape drive of the adjacent library storage module. Such a configuration provides advantages to managers of large amounts of data who are able to simply add additional library storage modules as their need for data storage grows.
As described above, an important component in the utilization of multiple library storage modules is the pass-through system, which removes a tape cartridge from a first library storage module and conveys the cartridge to a second library storage module. In many prior art methods, the pass-through system must change the orientation of the cartridge from that which is suitable for the first library storage module to that which is suitable for the second library storage module. Specifically, the unique identifying information for each tape cartridge, typically a bar code, is affixed to a face of the cartridge which protrudes from a cell when the cartridge is stored therein. Therefore, in the prior art, the pass-through system must remove a tape cartridge from a first library storage module at a first orientation and rotate the tape cartridge to a second orientation for the second and adjacent library storage module.
To achieve the operations described above, many prior art pass-through systems have required the use of motors, power amplifiers, sensors, and electrical cabling. While such prior art pass-through systems have been effective, they have suffered from problems of size, complexity, and cost. First, the use of motors and associated equipment requires the pass-through system to be relatively large. Since space is made for the pass-through system by removing cartridge cells from the cell arrays in each library storage module, the extra space required by the motors and associated equipment causes additional cartridge cells to be eliminated, thereby reducing the overall storage capacity of each library storage module. The motors, amplifiers, and sensors also increase the complexity of the pass-through system, making it more expensive to manufacture initially and more expensive to maintain over the life of the library storage module. An additional cost with regard to prior art pass-through systems is the need for a library management unit to interpret and convey commands between a host computer and the library control unit of each library storage module. A library management unit is necessary because signals must be supplied to the motors and sensors of the pass-through system during operation.
Commonly owned U.S. Pat. No. 5,700,125 issued to Falace et al., discloses a pass-through system wherein motors, amplifiers, and sensors are eliminated, and the force of gravity alone is used to move a tape cartridge from a cell in an originating library storage module, rotate the cartridge, and deliver the cartridge to a cell in a receiving library storage module. In operation, signals cause a robotic arm of the originating library storage module to present a tape cartridge to a designated entry cell in the originating library. The back of the entry cell opens to an entrance chute of a pass-through port, and the cartridge slides into the entrance chute by the force of gravity. A bucket mounted on the entrance chute is positioned to receive the cartridge, and the bucket pivots against the force of a biasing spring to present the cartridge to an exit chute of the pass-through port. The tape cartridge has now been turned so that the identifying label thereof is now facing inward to the receiving library storage unit module. As the bucket pivots, the cartridge slides out of the bucket, down the exit chute, and into an exit cell in the receiving library storage module.
Therefore, the pass-through system of U.S. Pat. No. 5,700,125 decreases the size, complexity, and cost compared with prior art pass-through systems. However, the reliance completely on gravity to provide the pass-through function does have disadvantages. Specifically, in order to provide complete interchangeability between two library storage modules, two pass-through systems are required in horizontally adjacent library storage modules: one pass-through system from the originating library module to the receiving library module, and a second pass-through system from the receiving library module back to the originating library module. Furthermore, the pass-through system is only capable of working in one direction in vertically adjacent libraries, such that cartridges that are passed to a downwardly adjacent library module can never be passed back upward. Lastly, the process by which the tape cartridge is transferred between library storage modules still requires the orientation of the cartridge to be changed in order to ensure proper library operation.